This invention is generally directed to liquid developer compositions and the improved developed images obtained thereof in electrographic image on image printing processes wherein a stylus provides, or writes the image pattern on a dielectric receptor, and more specifically, the present invention relates to a liquid developer containing certain charge control agents. More specifically, the present invention relates to positively charged liquid developers comprised of a nonpolar liquid, pigment, or dye, cyclodextrin charge control agent and a charge director, and which developers possess a number of advantages including the development and generation of images with improved image quality. The developers of the present invention in embodiments provide images with higher reflective optical density (ROD) and/or lower residual voltages (V.sub.out). Higher reflective optical densities provide images with deeper, richer desirable color or more extended chroma. Lower residual image voltages enable the printing of subsequently applied layers to a higher reflective optical density and decrease or eliminate image defects such as smearing and shifts in L.sup.* a.sup.* b.sup.* color space (hue shifts) when one colored layer is overlaid on a second layer of different color. Series-Capacitance Data was utilized as a means of measuring the total charge in the liquid developer formulation, and which measurements indicate that placing too much charge on the toner or developer particles can cause lower RODs to occur, which is a manifestation of inferior image quality because less chroma would occur. Moreover, there can be added to the liquid developers of the present invention in embodiments thereof charge directors of the formulas as illustrated in copending application U.S. Ser. No. 08/505,043, now U.S. Pat. No. 5,563,015, especially a mixture of Alohas and EMPHOS PS-900.TM., or Alohas alone, an aluminum-di-tertiary butyl salicylate, the disclosure of which is totally incorporated herein by reference.
The developers can discharge the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet. Also known are ionographic imaging systems. Insufficient particle charge can result in poor image quality and also can result in poor transfer of the liquid developer or solids thereof to paper or other final substrates. Poor transfer can, for example, result in poor solid area coverage if insufficient toner is transferred to the final substrate and can also cause image defects such as smears and hollowed fine features. Conversely, over-charging the toner particles can result in low reflective optical density images or poor color richness or chroma since only a few very highly charged particles can discharge all the charge on the dielectric receptor causing too little toner to be deposited. To overcome or minimize such problems, the liquid toners, or developers of the present invention were arrived at after extensive research, and which developers result in, for example, sufficient particle charge to enable effective transfer but not so much charge as to yield images with lower optical densities and lower residual voltages because of excess toner charge. An advantage associated with the present invention includes controlling the increase of the desired positive charge on the developer particles.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. These dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive imaging member or layer with a uniform electrostatic charge, and developing the image with a liquid developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 30 .mu.m (microns) average by area size as measured with the Malvern 3600E particle sizer.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors illustrated may include both negative charge directors, such as lecithin, oil-soluble petroleum sulfonates and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer (iii) of vinyl toluene and styrene and (iv) butadiene and acrylate. As the copolymer with polyethylene and methacrylic acid or methacrylic acid alkyl esters, NUCREL.RTM. may be selected.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner pigmented particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers can be prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture whereby the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge adjuvant. Liquid developers with charge directors are also illustrated in U.S. Pat. No. 5,045,425. Also, stain elimination in consecutive colored liquid toners is illustrated in U.S. Pat. No. 5,069,995. Further, of interest with respect to liquid developers are U.S. Pat. Nos. 5,034,299; 5,066,821 and 5,028,508, the disclosures of which are totally incorporated herein by reference.
Lithographic toners with cyclodextrins as antiprecipitants, and silver halide developers with cyclodextrins are known, reference U.S. Pat. Nos. 5,409,803, and 5,352,563, the disclosures of which are totally incorporated herein by reference.
Illustrated in U.S. Pat. No. 5,306,591 is a liquid developer comprised of a liquid component, thermoplastic resin; an ionic or zwitterionic charge director, or directors soluble in a nonpolar liquid; and a charge additive, or charge adjuvant comprised of an imine bisquinone; in U.S. Statutory Invention Registration No. H1483 a liquid developer comprised of thermoplastic resin particles, and a charge director comprised of an ammonium AB diblock copolymer, and in U.S. Pat. No. 5,307,731 (a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid, the disclosures of each of these patents being totally incorporated herein by reference.
In copending patent application U.S. Ser. No. 986,316, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., the developer having a melting point of at least about 25.degree. C., wherein contacting occurs while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.